(a) Field of the Invention
The present invention relates to a lens tube structure of a multiple lens group type high vari-focal ratio zoom lens and, more particularly, to the focusing mechanism of a zoom lens arranged to carry out focusing by moving lens groups including at least one lens group other than a first lens group, which is arranged on the object side, said focusing mechanism being arranged to automatically compensate the movement amounts of said focusing lens groups which vary according to zooming.
(b) Description of the Prior Art
In known zoom lens systems, functions are alloted to the lens groups in such way that there is a first lens group for zooming and a second lens group for focusing and, generally, the first lens group arranged on the object side is used for focusing. For this purpose, the lens mount of the first lens group is arranged to be moved forward and backward in respect to the support tube thereof, by means of a helicoid, cam, etc., according to rotation of the focusing ring. In case of a two-group zooming lens system arranged to carry out focusing by varying an airspace between two lens groups, the first lens group is moved for both purposes of zooming and focusing. In this case, however, it is satisfactory if the lens moving mechanism is arranged so that the support tube is moved for the purpose of zooming and the lens mount for the first lens group is moved for the purpose of focusing. Therefore, the lens moving mechanism is substantially the same as the case of the aforementioned known zoom lens systems.
This is because the position of the conjugate point of the object in respect to the principal point on the image side of the first lens group may be regarded as approximately constant even when the lenses are moved for the purpose of zooming and, therefore, when the lens system is once focused, it is kept in the focused state even when zooming is carried out after focusing.
In case of known lens tubes for this kind of zoom lens systems, movement of lens groups for the purpose of zooming is generally effected by means of a single helicoid or combination of a cam and slide tube. As, however, the varifocal ratio becomes larger, the cam mechanism becomes complicated and, at the same time, problems are caused in reliability.
Some of recently disclosed zoom lenses are arranged so that the first lens group is moved by using a double helicoid mechanism in its case where the movement amount of the first lens group is large.
When, however, only a particular lens group is used for focusing as described above, the movement amount of the lens group becomes large when it is attempted to focus the lens system on an object at a short distance. Moreover, in case of a zoom lens which contains a wide angle in its vari-focal range, the diameter of the first lens group should be made large in order to prevent decrease in the quantity of marginal rays, and this makes it difficult to arrange the lens system compactly and to improve simplicity of operation.
On the other hand, when the functions of respective lens groups are not fixed as in case of known zoom lenses but it is so arranged that the lens groups having the functions for zooming are utilized also for focusing, the number of lens groups which can be utilized for zooming increases even when the lens configuration of the zoom lens system is not changed and, consequently, it is possible to increase the zoom ratio. Moreover, the number of lens groups which can be utilized for focusing also increases and, therefore, it is possible to focus the lens system up to a short object distance without making the movement amounts of lens groups large. A new type of zoom lens system arranged as described above is also developed.
However, in case of the above-mentioned type of zoom lens system, optical conditions such as the total focal length of focusing lens system, positions of conjugate points, etc. changed according to zooming. Therefore, even when the zoom lens system is once focused, it cannot be maintained in the focused state unless lens groups are further moved for focusing when the lens system is zoomed. Moreover as the movement amounts at that time vary according to changes of two variables, i.e., the distance to the object and focal length of the zoom lens system, a two-dimensional cam is generally required and it is practically impossible to arrange such lens moving mechanism into a small-sized portable zoom lens.
On the other hand, it is now made clear that zooming by keeping the lens system focused on the same object is attained by moving the focusing lens groups using different portions of only one curved cam, utilizing the fact that the curve of lens movement amounts required for zooming becomes a curve resembling a logarithmic curve when the object distance is regarded as constant.